Abstract
Striated muscle tissue contains fibers with high oxidative capacity (heart muscle), higher oxidative capacity (type I and IIA fibers of skeletal muscle) and low oxidative capacity (type IIB/X fibers of skeletal muscle). Muscle fibers with higher oxidative capacity contain large mitochondria tightly packed with cristae as well as small forms of mitochondria containing relatively few cristae. The intensive development of the mitochondrial apparatus in the post-activity period reflects the adaptive processes, which is intended to supply the increased energy requirements of muscle fibers with higher oxidative capacity. Muscle fibers with low oxidative capacity contain significantly less mitochondria than fibers with higher capacity. It is typical to type IIB fibers that after intensive muscle activity there are damaged myofibrils in a relatively small area, some myofibrils are twisted and lose the connection with the neighboring structures. It is still not fully known how skeletal muscles with different oxidative capacity respond to an increased functional activity and what differences exist in these fibers between oxidative capacity and function of myofibrils. The aim of the present short review was to compare structural-functional changes in mitochondrial and myofibrillar compartments of heart and skeletal muscle fibers with different oxidative capacity and the effect of increased functional activity on the interaction of these compartments.
Highlights
Skeletal muscle fibers with higher oxidative capacity are relatively small compared to fibers with low oxidative capacity (Figure 2 and Figure 3), pointing to an increase in relationships between fiber cross-sectional area (CSA) and VO2max [30]
The higher degradation rate of protein in type I and IIA muscle fibers is balanced by a high rate of protein synthesis
In muscle fibers with high or higher oxidative capacity intracellular phosphotransfer system constitutes a major mechanism linking the mitochondria and ATPases. In these fibers mitochondria are precisely positioned between the myofilaments, muscle fibers are small compared to fibers with low oxidative capacity and show the relationships between fiber CSA and VO2max
Summary
During low intensity and long-lasting activity slow-twitch (ST) and fast-twitch (FT) type IIA muscle fibers are mainly recruited These fibers have higher oxidative capacity and ST fibers are fatigue resistant. Small structural rearrangements take place in type IIX/IIB muscle fibers during low intensity muscle activity as these fibers are recruited less. During the process of adaptation to the intensive functional activity it is typical to type IIB/X fibers that there are damaged myofibrils in a relatively small area, some myofibrils are twisted and lost the connection with the neighboring structures [13]. Changes induced by intensive muscle activity (increased workload) at the muscle-fiber level are related to hypertrophy of IIA and IIB/X fibers. Muscle activity may induce complete and focal destruction of some fibers, partial denervation of fibers, and these may be the factors for the activation of satellite cells [25] [26]
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